Page 74 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Key Terms

SUMMARY

There are three basic crystal structures in metals: body-centered cubic (bcc), face-
centered cubic (fcc), and hexagonal close-packed (hcp). Grains made of these
crystals typically contain various defects and imperfections, such as dislocations,
vacancies, impurities, inclusions, and grain boundaries. Polycrystalline metals
consist of many crystals, or grains, in random orientations.
Plastic deformation in metals takes place by a slip mechanism. Although the
theoretical shear stress required to cause slip is very high, actual stresses are
much lower because of the presence of dislocations (edge or screw type).
Dislocations become entangled with one another or are impeded by barriers
such as grain boundaries, impurities, and inclusions. As a result, the shear stress
required to cause further slip is increased; consequently, the overall strength
and hardness of the metal is also increased (through work hardening or strain
hardening).
Grain size has a significant effect on the strength of metals: The smaller the size,
the stronger is the metal, and the larger the size, the more ductile is the metal.
Grain boundaries have a major influence on the behavior of metals. Boundaries
can undergo embrittlement, severely reducing ductility at elevated temperatures
(hot shortness); they are also responsible for the creep phenomenon, which is due
to grain boundary sliding.
Metals may be plastically deformed at room, warm, or high temperatures, their
behavior and workability depending largely on whether deformation takes place
below or above the recrystallization temperature of the metal. Deformation at
room temperature (cold working) results in higher strength, but reduced ductility;
generally, it also causes anisotropy (preferred orientation or mechanical fibering),
whereby the properties are different in different directions.

The effects of cold working can be reversed by annealing the metal: heating it
in a certain temperature range for a given period of time, thereby allowing
the successive processes of recovery, recrystallization, and grain growth to take
place.

KEY TERMS

Allotropism Grain boundaries Orange peel Slip system
Anisotropy Grain growth Plastic deformation Strain hardening
Basal plane Grain size Polycrystals Structure-insensitive
Body-centered cubic Hexagonal close-packed Polygonization Structure-sensitive
Cold working Homologous temperature Polymorphism Texture
Covalent bond Hot shortness Preferred orientation Twinning
Creep Hot working Primary bond Unit cell
Crystals Imperfections Recovery Vacancy
Dislocations Ionic bond Recrystallization van der Waals force
Elastic deformation Lattice structure Secondary bond Warm working
Embrittlement Mechanical fibering Shear stress Work hardening
Face-centered cubic Metallic bond Slip band
Grains Nucleation Slip plane